Add FREEBSD_CC_VERSION which will be used to define __FreeBSD_cc_version.

The WITH_SYSTEM_COMPILER build option will rely on this value to determine what
__FreeBSD_cc_version the source tree will produce. This value will be compared
against the /usr/bin/cc value to determine if a new compiler is needed.

Start with 1100002 which is 1 more than than the value we've had since
3.8.0 to ensure that all changes since then are present.

Reviewed by: dim
Sponsored by: EMC / Isilon Storage Division